1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
As a nonvolatile semiconductor memory device, a floating gate type memory device with a conductive nano-particle layer provided between tunnel insulation films is proposed (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-289710). One example of a conventional floating gate type memory device will be described as follow.
A lower tunnel insulation film, a nano-particle layer, and an upper tunnel insulation film are successively formed on a silicon substrate having a source/drain region, and a charge storage portion which serves as a floating gate electrode is formed on the upper tunnel insulation film. Furthermore, a control insulation film and a control gate electrode are successively formed on the charge storage portion. The nano-particle layer is formed by conductive nano-particles which satisfy the Coulomb blockage condition (charging energy of one electron is greater than thermal fluctuation). For the charge storage portion, for example, a silicon nitride film (Si3N4 film) is used.
Information is written by injecting electrons into traps in the silicon nitride film (charge storage portion). That is, by applying the positive voltage to the control gate electrode, carrier electrons in the inversion layer formed in the silicon substrate surface are injected into traps in the silicon nitride film via the tunnel oxide films with the nano-particle layer held therebetween. Information is read by discriminating quantity of drain current that complies with presence or absence of trap charges. Trap charges are discharged by allowing the trap charges to tunnel to the silicon substrate via the tunnel oxide films with the nano-particle layer held therebetween by applying negative voltage to the control gate electrode.
However, in the conventional structure as described above, there has been a problem that percolation leakage between the source and drain generated by device size scaling cannot be successfully suppressed. Consequently, it has been difficult to obtain a semiconductor device which achieves excellent characteristics and reliability.